Electrolytic manufacture of manganates and/or permanganates



Dec. 20, 1966 c. A. MAzzUcHELLx ETAL 3,293,150

ELECTROLYTIC MANUFACTURE OF MANGANTES AND/OR PERMANGANATES Filed Dec.19, 1962 2 Sheets-Sheet 1 Y: G. L

IFJGZ AT TO 2N EYS Dec. 20, 1966 c. A. MAzzUcHELLl lair/xx. 3,293,160

ELECTROLYTIC MANUFACTURE OF MANGANATES AND/OR PERMANGANATES 2Sheets-Sheet 2 Filed Dec. 19, 1962 P M U P CQYSTUZER SOLOBHJTY HJ 400UTER w. wo awr] Fi@ INVENTORS Cnnes A. MAzzUcHELm Yrosevn Snmomoas leomo g 7" 'rampen-maa P) ATTORNEYS United States Patent O 3,293,160ELECTROLYTIC MANUFACTURE OF MANGA- NATES AND/R PERMANGANATES Charles A.Mazzuchelli, La Salle, lll., and Joseph Samonides, Tipton, Tenn.,assignors to E. J. Lavino and Company, Philadelphia, Pa., a corporationof Delaware Filed Dec. 19, 1962, Ser. No. 245,934 1 Claim. (Cl. 2014-82)This invention 'relates generally to the manufacture of manganatesand/or permanganates.

An object of the present invention is to produce manganates and/orpermanganates by electrolysing an alkaline solution, using pure, orsubstantially pure manganese metal as the anode.

More particularly the invention has for one of its objects, theadaptation of a cathode from an electrolytic managanese producingprocess, to use as an anode in an electrolytic cell for producingmanganates and/or permanganates.

Still another object of the invention is to provide a new process ofproducing manganates and/or permanganates by electrolysing an alkalinesolution in which the anode is pure or substantially pure manganesemetal deposited upon an electric current conducting core and which anodehas been formed or produced as the cathode of an electrolytic manganeseproduction process.

A still further object of the invention is to provide a novel process ofproducing a permanganate in concentrated solution in an anolyte andrecovering crystals of the permanganate directly from such solution.

Still another object is to provide a novel process of producing amanganate in concentrated solution in an anolyte.

In connection with the present invention it has been found that by theuse of the combination of an anode of pure manganese and a cathode, inan alkaline solution in accordance with the presen-t process, manganatesand/ or permanganates can be produced directly from the anolyte. In suchoperation the running of the electrolytic cell for a predetermined andsufficiently long period will build up a concentration of the manganateor permanganate in solution to a point where recovery of these compoundsdirectly from the solution can be effected.

In the accompanying drawings the basic elements necessary for theproduction of the manganates and/ or permanganates, in accordance withthe present invention, are illustrated, together with details of suchelements and a ow diagram by which to carry out the invention as acontinuous process.

In the drawing FIG. l illustrates, in longitudinal section, a suitabletype of cell embodying a single anode and a diaphragm or membraneshielded single cathode therein by means of which the Working of theinvention may be realized.

FIG. 2 is a View in top plan of the cathode structure and an enclosingor encasing diaphragm and showing one end only ofthe cell.

FIG. 3 is a view taken substantially on the line 3 3 of FIG. 2 and on anenlarged scale.

FIG. 4 illustrates a ow diagram for the continuous production ofpermanganates in accordance with the present invention.

FIG. 5 is a gra-ph illust-rating the eifect of temperature on potassiumpermanganate solubility in an anolyte solutlOIl.

Referring more particularly to FIGS. 1 to 3 of the drawings, the numeralgenerally designates a single cell suitable for carrying out the presentinvention and which cell may be formed or constructed of any suitablematerial to contain the selected anolyte solution and inert to orunaffected by such solution.

lthe present manganates or permanganates.

3,293,160 Patented Dec. 20, 1966 Any suitable provision may be made forcontrolling the cell temperature such as by a bath of flowing water,when necessary, or the cell or a number of cells may be operated at roomtemperature without cooling, where such procedure is found feasible.

In large scale operation of the invention, provision may be made forcontinuously removing crystals of the formed compounds as they developin the anolyte, as by constructting the production cell in aconventional method, such, for example, as by forming the bottom of thecell in a V. Another means of removing crystals might be by providing asuitable channel to allow for the use of a screw conveyor to be mountedin the cell bottom or suitable arrangement may be made in the operationof the cell, as in the hereinafter described system, to remove thecrystals as they are formed or suspended in the supersaturated solutionto another tank or receptacle to be allowed to settle out or separatedfrom the solution in any other suitable manner.

The method of conducting the electrolyzing current through the anode,anolyte and to the cathode, or through the hereinafter describeddiaphragm to the cathode, may be of any suitable character mostconvenient and economical to the installation and operation of thesystem, and, therefore, is not to be limited to any special type ofwiring, etc. Additionally, the cell 10 may be of any desired size orform determined by requirements of the installation and the positions ofthe anode and cathode are only limited with respect to the relation ofthe same one to the other.

The numeral 12 generally designates the anode which is suspended in thecell and in the anolyte solution and which solution is generallydesignated 14.

For the production of the manganates or permanganates according to thepresent invention, the anode in the form here illustrated, or in anysuitable form, consists of pure, or substantially pure manganese metalattached to and supported by a suitable electric current conductingbody.

In addition to the foregoing the anode may be in the form of a solidplate of manganese metal or in chips or pie-ces of the manganese metalin or on a suitable carrier and the anode, of course, forms the positivepole of the circuit in which the electrolytic cell is connected.

The solid manganese electrode may also be produced by the process ofpowder metallurgy.

It is important in any case that the construction of the manganese bodyand its connection with the electric current conductor be such that thecurrent be conveyed into and through the entire mass of the manganesemetal into the electrolyte solution.

The anode 12 in the preferred structural form, -as illustrated in FIG.l, comprises an elongate core 16 of a suitable current conductingmaterial, which may be a metal or non-metal such as titanium, iron,stainless steel, or carbon, upon which the manganese metal, designated18, is plated.

While the manganese metal may be applied to the core orvcarrier body 16,as illustrated, in actuality, the anode here shown and in the preferredform or embodiment, is formed as the cathode from the electrolyticmanganese process, adapted to the present invention as the anode in theelectrolytic cell for the production of l In other words, in the processof electrolytically producing the manganese metal, the cathode uponwhich the metal is deposited, is used in the present process ofproducing a manganate or `a permanganate, as the positive pole in theelectrolytic cell.

Any suitable means may be employed for suspending the anode in the calland in the electrolyte. The means here illustrated may comprise a bridgepiece of metallic bar 26, preferably of copper.

or non-metallic material extending across the top of the cell and havingthe core of the anode fixed thereto in any suitable manner. The core ishere illustrated as extending through the bridge piece and carryingsuitable means for attaching an electric current conductor 20 thereto.

The numeral 22 generally designates a cathode unit consisting of thecathode element and an enclosing shield or diaphragm.

The cathode element is designated 24. While this element 24 may beconstructed in any suitable manner and any suitable material, it is hereillustrated as being in the preferred form of a screen or grid supportedfor suspension in the cell and for extension into the catholyte.

The cathode element 24 is not, of course, restricted in any way, as toits form since it may be in the form of a mesh screen or grid, asstated, or a plate or rod and, of course, of any material eithermetallic or nonmetallic will conduct the required current load.

The cathode element and the unit as a whole, also is not limited in sizebut, as hereinafter set forth, there is a prefer-red minimum size ratiobetween the surface areas of the anode and the cathode element belowwhich the most desirable results in terms of economical operation kofthe cell, are not realized.

Acathode unit or structure suitable for working the present invention ishere illustrated as embodying a metal The bar 26 is positioned betweenand has secured thereto the upper ends of the vertical legs of twosubstantially U-shaped frames 28. These vertical legs of each of theframes, are designated 30 and the lower ends of the legs 30 areconnected by the horizontal portions 32.

Any suitable means may be employed for securing the upper ends of thevertical legs 30 of the frames 28, to the bar 26 located therebetweenand such means is here shown as a pin or bolt 34 passing transverselythrough the legs and through the metal bar.

The cathode element 24 is electrically attached or joined to the metalbar 26 to be supported thereby, as hereinafter described, so as todepend into the cell and into the anolyte. When in the form of a screenor grid as here particularly illustrated, such screen or grid may have aconstruction to be draped over the bar 26 to provide the two sideportions or walls 38 and when so draped over the bar 26 it will belocated within the area defined by the frames 28 as best seen in the topview of the unit, forming FIG. 2. Thus it will be seen that when the bar26 is positioned `across the top of the cell, the grid will extenddownwardly into the alkaline solution forming the catholyte. It will, ofcourse, be understood that when a cathode element of a form other than agrid or screen such as that illustrated, is employed, it would be lixedor electrically coupled in a suitable manner to the bar to extenddownwardly therefrom when the bar is placed in operative position overthe top of the cell.

In the illustrated construction, the open mesh metal structure formingthe cathode element when draped over the bar 26, may have a spacer 40interposed between the sides 38 thereof and `at the lower or bottom endsof such sides, as illustrated in FIG. 3.

In the construction of the cell as designed for the manufacture ofpermanganates, a separating diaphragm Yis interposed between the anodeand the cathode element.

This separating diaphragm is here generally designated 42 and in theconstruction illustrated the diaphragm forms a casing around the cathodeelement and the diaphragm material extends around the outer sides of theframes 28 and across the bottom elements 32 of the frames thus forming achamber or enclosed area which is designated 44.

The diaphragm is secured to the frames in any suitable manner, as, forexample, by the provision of holding strips 46 between which and thevertical and horizontal members of the frame 28, the diaphragm materialis secured.

While the casing or diaphragm material may be placed around the cathodeelement in contact therewith, it is preferred that it be spaced :fromthe same so as to allow for the free escape of gases which may beevolved in connection with certain reactions which take place in theenclosed area 44, as hereinafter set forth.

The cathode unit may be mounted in any suitable manner whereby to permitthe cathode element and the enclosing diaphragm to depend into thealkaline solution 14 of the cell. The unit is supported so that the topof the diaphragm encasing the cathode element projects above the levelof the solution. In the arrangement here illustrated, the bar 26 isdisposed across the top of the cell and rests at its ends upon the sidewalls of the cell above which it may Ibe insulated, if necessary.

Any suitable means may be provided for connecting the bar with anelectric current conducting wire forming a part of an electric circuitin which the cell is connected and whereby electrical energy in the formof direct current may be caused to pass from one electrode to the otherthrough the solution. Any suitable means may be provided for furnishingthe direct current at the required voltage and amperage.

While a current conductor 36 is here illustrated as attached to an endof the bar 25, this is merely for the purpose of completing thedisclosure and is not intended to be in any way limiting as tothe mannerin which the bar 26 may be connected in the electric current.

The diaphragm is, of course, formed of a lsuitable material which isinert as regards any reaction with the alkaline solution and it may bein the form of a porous synthetic resin foam, cloth, porous ceramic,asbestos fabric or paper, board, or the like. Examples of syntheticresin foam or liber material which may be employed are polypropylene,polyethylene, and acrylic resins.

The alkaline solution used in the present process of forming theanolyte, may comprise or consist of an alkali or alkaline earth saltwhich can be electrolyzed. As an example, and without intending to inany way limit the invention, the anolyte may consist of a potassiumcarbonate solution, a sodium carbonate solution or a solution of anyother alkaline earth salt or a mixture of such salt or salts and ahydroxide lsuch as potassium hydroxide, all, of course, aqueoussolutions.

The Vfollowing example is -given of the use of potassium carbonate inthe production of a permanganate in accordance with the presentinvention.

Example I An anolyte solution was prepared consisting of approximately400 grams per liter of potassium carbonate in aqueous solution andplaced in the cell 1t), in which the manganese metal anode formed byelectrolytic deposition on Va core of titanium, was suspended, togetherwith a suitable cathode element and an encasing diaphragm, alsosuspended in the solution, substantially as illustrated in FIG. l. Acathode element consisting of a wire grid and using a synthetic iberdiaphragm was employed. The anode to cathode element ydistance was about3 inches and energy was applied at the rate of 5.5 volts and 133 amps.per square foot for a period of about 168.5 hours. At the end of the168.5 hour period about 91.8 lbs. of potassium permanganate wereproduced. The yield of this product based on the manganese metalk inaccordance with this example is approximately 97%.

The ratio of the anode surface area in solution to the cathode elementsurface area in solution is approximately 1 to 2 with electrical energyabout 14 amps. per square decimeter. Electrical eiiiciency isapproximately 32.5%.

For the preparation of manganates the potassium carbonate solution maybe replaced by a solution of potassium hydroxide in accordance with thefollowing example:

Example II The production of manganates in accordance with the presentinvention may be effected without the use of the diaphragm interposedbetween the anode and the cathode element. In this example an anolytesolution was prepared consisting of approxi-mately 1000 grams per literof potassium lhydroxide in aqueous solution and placed in the cell 10,in which were suspended the manganese metal anode formed by electrolyticdeposition on a core of titanium, and a suitable cathode element asillustrated in FIG. 1, but without the diaphragm. A cathode elementconsisting of a wire mesh grid was used. The anode to cathode elementdistance was about 3 cms. and energy was applied at the rate of about 3volts and 13 amperes for a period of about 5 hours. At the end of thefive hour period about 7.3 .grams of potassium manganate were lproduced.The yield of this product based on the manganese metal in accordancewith this example, is approximately 57 percent.

The ratio of the anode surface area in solution to the cathode elementsurface area in solution is approximately l, to 2.1/2 with electricalenergy about 40 amperes per square decimeter. The electrical efficiencyis approximately 13 percent.

The operation of the cell is such that the cell may be made up of aseries of alternate anodes, diaphragms, and cathode elements which canbe operated as a cell in series or in parallel. There is no limitationto the number of such anode, diaphragm, and cathode element combinationswhich may be employed. Neither is the current density of electricalenergy applied to the anode limited, but can vary from as low as 2amperes to 50 amperes or more persquare decimeter.

The energy applied to the cathode may also vary accordingly.

It is found that power requirements for operation of the cell vary inaccordance with the ratio of the surface area of the anode to thecathode element. For example, if the ratio of the surface area of theanode to cathode elements is as 1 to 2, the power requirements remain atone factor. If the ratio of the anode as 1 is varied to the cathodeelement of say 4 times as great, the power requirement or energynecessary for production of the end product will be less. If the cathodeelement ratio is reduced where the ratio between the anode and cathodeelement is say l to 1 or 1 to 1A, the power requirements increaseproportionately and seem to be in direct ratio to the reduction of thecathode element area to the anode area.

It is also found that the concentration of the anolyte may be as low as100 grams per liter and as high as its saturation point withoutimpairing the operation of the cell.

power requirements increase while they decrease as the temperaturerises.

In addition to the fact that the concentration of the anolyte may varyfrom 100 to the saturation point of the salt or hydroxide per liter ofaqueous solution, the anode to cathode element ratio may vary from 1 to1A to 1 to 8 or more.

Satisfactory results are obtained with a low current density and a highcurrent density and it has been found that the current density on theanode may vary from as low as 2 amps. to as high as 50 amps. or more persquare decirneter.

The main reaction which takes place in the cell as a whole may be shownas:

At the cathode the evolution of hydrogen occurs and this, of course, iswell recognized as elementary in electro- 6 lytic dissociation ofaqueous solution. At the anode, in addition to the oxidation of the Mnto the MnO4, other reactions take place. The principal side reaction inthe cell of the present invention is the formation of possible oxides ofmanganese and the evolution of oxygen.

In the operation of the cell potassium hydroxide builds up in thecatholyte solution and the addition of water or a weak solution ofalkali is required to get the potassium hydroxide out and neutralize thebicarbonate to carbonate.

To keep the potassium at the same level in the cell system and preventthe accumulation of bicarbonate equivalent to the potash used in makingthe permanganate, potassium hydroxide is added. Thus for every 15.8 lbs.of K MnO., produced in the anolyte, 5.6 lbs. of KOH should be added tothe anolyte solution to keep the solution in balance. This KOH should beadded in a concentrated solution so as to keep down the anolytedilution.

FIG. 4 illustrates a flow system for producing permanganate crystals inaccordance with the present invention, while FIG. 5 is a graphillustrating the effect of the temperature on potassium permanganatesolubility in the anolyte solution based on approximately 400 grams ofpotassium permanganate in the aqueous solution.

The graph illustrates the approximate operating temperature for the cellwhich will produce in the cell solution the approximate number of gramsof potassium permanganate per liter `of solution without crystalsforming in the cell. In other words, so long as the cell temperature ismaintained for a solution of the concentration indicated, at anindicated degree on the curve, the permanganate will remain in solutionand crystals will only be formed by lowering the temperature.

In one run of the process, in a potassium carbonate solution of 600grams per liter of water, approximatelyl 16 grams of the permanganateform and may be extracted at a solution temperature of 65 C. while at a70 C., approximately 19 grams per liter of the permanganate areobtained.

Thus operating the cell at say approximately 70 C., when theconcentration of permanganate in the cell reaches less than 19 grams perliter, allowing the liquid to run into a cooling tank and reducing thetemperature of from about 10 to 20, will cause a good part of thecrystals to form and drop out. However, if the cell is operated at thistemperature, then the make-up solution tank, hereinafter described,should be heated to this point to keep remaining crystals in solution,the idea being to form the crystals only in the hereinafter describedcooling tank.

Referring now to FIG. 4 the numeral 50 generally designates the make-uptank in which the selected alkali is dissolved in water, which alkaliis, or may be potassium carbonate, as hereinbefore stated, or any othersuit: able alkaline earth salt. In this make-up tank, which is providedwith a suitable heating means 52 and agitator 53, the temperature of thesolution is raised to the desired degree. The anolyte prepared in themake-up tank flows therefrom as diagrammatically illustrated, into thecell here generally designated 10, as in FIG. 1, where the hereinbeforedescribed electrolytic action is carried out.

The numeral 54 designates a catholyte solution or catholyte make-up andsupply tank for addition of catholyte to the cathode element cells whichform a part of the ,hereinbefore described unit generally designated22.1 The catholyte in this tank may comprise a weak alkaline solution ormay be water.

The numeral 54a designates a receptacle in which a concentratedpotassium hydroxide solution is maintained to be fed as required intothe system for mixture with the crystals and liquor passing oilC fromthe reaction cell 10 into the cooling and crystallizing tank 55.

As hereinbefore stated the addition of potassium hydroxide to theanolyte is required to keep the potash at the same level in the cellsystem-or in other words to replace the potassium ions removed in theformation of the MnO4. This potassium hydroxide is maintained inconcentrated solution in the tank 54a whereby the addi-` tion of thealkali solution will be effective to prevent the accumulation ofbicarbonate in the cell Without diluting the solution.

From the reaction cell 10 to the liquor containing the potassiumpermanganate flows into the cooling and crystalliz'ing tank generallydesignated 55. After cooling in this tank where the liquor is reduced tothe proper degree to effect the formation of the permanganate crystals,the crystals are removed to a centrifuge 56 and the mother liquor isreturned by Way of the line 57 to the make-up tank 50.

The crystals from the centrifuge are passed into a tank of Water,generally designated 58, which is heated to about 90 C., and thecrystals are there agitated to ensure complete solution of the same. Inthis solvent tank crystals are added to a sufficient extent to produce asolution having a specific gravity of about 1150-ll60. This solutionthen passes from the solvent tank 58 to a filter 60 from which it ows asindicated into a crystallizer or receptacle 61 for effecting crystalformation.

In the crystallizer the liquor is allowed to stand without agitation forat least 48 hours for crystals to build up.

The mother liquor extracted in the centrifuge passes from the centrifugeto a receiving tank 62 from which it returns by a line 63 to the anolytetank 50.

After the crystals are formed, the mother liquor is drawn off andrecycled either to the dissolver 58 as shown or it may be returned tothe anolyte make-up tank as desired.

The crystals removed from the crystallizer 61 are centrifuged at 64 andthe mother liquor is passed from the centrifuge and recycled to thedissolver or the anolyte tank While the crystals are sent to the dryer65.

As this invention may be embodied in several forms Without departingfrom the spirit or essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claim rather than bythe description preceding them, and all changes that fall within themetes and bounds of the claim or that form their functional as well asconjointly cooperative equivalents, are therefore intended to beembraced by this claim.

We claim:

The process of making a permanganate which comprises providing anelectrolyte make-up and supply tank with a solution of an alkali metalssalt selected from the group consisting of sodium carbonate andpotassium carbonate, heating said solution to about 65 to 70 C.,conducting the heated solution into an electrolytic cell,

electrolytically oxidizing an anode body consisting of substantiallypure manganese metal suspended in said heated electrolyte solution insaid electrolytic cell, suspending a cathode in the said electrolyte,passing a direct current through the said electrolyte in said cell,providing diaphragm a means in the cell for preventing the electrolytefrom directly contacting the cathode while electrical energy flows tothe cathode, thereby forming permanganate in the anolyte, drawing offthe anolyte from the cell into a cooling receiver, adding a concentratedsolution of potassium hydroxide .to the anolyte as the anolyte oWs tosaid cooling receiver to thereby prevent accumulation of bicarbonateWithout diluting the solution and to replace potassium ions removed byformation of permanganate, reducing the temperature of the anolytesolution in the cooling receiver from about l0 to 20 to effect formationand precipitation of permanganate crystals, then removing the crystalsfrom the receiver to a centrifuge and returning the mother liquorobtained from the cooling and crystallizing step to the said make-uptank, then centrifuging the permanganate crystals to remove the adheringmother liquor and returning the centrifuged liquor to the make-up tankand -from the make-up tank to the electrolytic cell, dissolving thecentrifuged crystals in Water, adding crystals from the centrifuge tothe solution of dissolved crystals to give the solution a specificgravity of abouty 1150-1160, filtering the latter solution, theneffecting crystallization of the permanganate from the said filteredsolution, re-using the solvent for dissolving additional crystals,centrifuging the crystals obtained from the filtered solution to removeall of the solvent therefrom, and finally Adrying the crystals.

References Cited by the Examiner UNITED STATES PATENTS y 1,281,08510/1918 Shoeld 204-82 1,291,680 l/ 1919 Lovelace et al. 204-82 1,360,70011/1920 Wilson et al 204--82 3,055,811 9/1962v Ruff 204-98 FOREIGNPATENTS 51,390 7/ 1937 Russia.

OTHER REFERENCES Soobshcheniya Akad. Nauk Gruzin S.S.R., volume 19, No.3, 285-91 (1957), Chem. Abst. 52:18022i, Chem. Abst. 33167329.

JOHN H. MACK, Primary Examiner.

MURRAY A. TILLMAN, Examiner.

L. G. WISE, H. M. FLOURNOY, Assistant Examiners.

1. THE PROCESS OF MAKING A PERMANGANATE WHICH CIMPRISES PROVIDING ANELECTROYTE MAKE-UP AND SUPPLY TANK WITH A SOLUTION OF AN ALKALI METALSSALT SELECTED FROM THE GROUP CONSISTING OF SODIUM CARBONATE ANDPOTASSIUM CARBONATE, HEATING SOLUTION ABOUT 65 TO 70*C., CONDUCTING THEHEATED SOLUTION INTO AN ELECTRLYTIC CELL, ELECTROLYTICALLY OXIDIZING ANANODE BODY CONSISTING OF SUBSTANTIALLY PURE MANGANESE METAL SUSPENDED INSAID HEATED ELECTRLYTE SOLUTION IN SAID ELECTRLYTIC CELL, SUSPEMDING ACATHODE IN THE SAID ELECTRLYTE, OASSING A DIRECT CURRENT THROUGH SAIDSAID ELECTROLYTE IN SAID CELL, PROVIDING DIAPHRAGM A MEANS IN THE CELLFOR PREVENTING THE ELECTRLYTE FROM DIRECTLY CONTACTING THE CATHODE WHILEELECTRICAL ENERGY FLOWS TO THE CATHODE, THEREBY FORMING PERMANGANATE INTHE ANOLYTE, DRAWING OFF THE ANOLYTE FROM THE CELL INTO A COOLINGRECEIVER, ADDING A CONCENTRADED SOLUTION OF POTASSIUM HYDROXIDE TO THEANOLYTE AS THE ANOLYTE FLOWS TO SAID COOLING RECEIVER TO THEREBY PREVENTACCUMULATION OF BICARBONATE WITHOUT DILUTING THE SOLUTION AND TO REPLACEPOTASSIUM IONS REMOVED BY FOR MATION OF PERMANGANATE, REDUCING THETEMPERATURE OF THE ANOLYTE SOLUTION IN THE COOLING RECEIVER FROM ABOUT10 TO 20* TO EFFECT FORMATION AND PRECIPITATION OF PERMANGANATTECRYSTALS, THEN REMOVING THE CRYSTALS FROM THE RECEIVER TO A CENTRIFUGEAND RETURNING THE MOTHER LIQUOR OBTAINED FROM THE COOLING ANDCRYSTALLIZING STEP TO THE SAID MAKE-UP TANK, THEN CENTRIFUGING THEPERMANGANATE CRYSTALS TO REMOVE THE ADHERING MOTHER LIQUOR AND RETURNINGTHE CENTRIFUGED LIQUOR TO THE MAKE-UP TANK AND FROM THE MAKE-UP TANK TOTHE ELECTRLYTIC CELL, DISSOLVING THE CENTRIFUGED CRYSTALS IN WATER,ADDING CRYSTALS FROM THE CENTRIFUGE TO THE SOLUTION OF DISSOLVEDCRYSTALS TO GIVE THE SOLUTION A SPECIFIC GRAVITY OF ABOUT 1150-1160,FILTERING THE LATTER SOLUTION, THEN EFFECTING CRYSTALLIZATION OF THEPERMANGANATE FROM THE SAID FILTERED SOLUTION, RE-USING THE SOLVENT FORDISSOLVING ADDITIONAL CRYSTALS, CENTRIFUGING THE CRYSTALS OBTAINED FROMTHE FILTERED SOLUTION TO REMOVE ALL OF THE SOLVENT THEREFROM, ANDFINALLY DRYING THE CRYSTALS.